This invention relates to the process of forming flat glass known as the float process, wherein molten glass is delivered onto a pool of molten metal, and attenuated to a ribbon of desired thickness. Glass entering the float forming chamber or "bath" is initially at a relatively high temperature, e.g., around 2000.degree. F. (1100.degree. C.), so as to be relatively fluid, but during its passage along the float chamber, the glass must cool to a condition suitable for engagement with attenuating devices, e.g., around 1400.degree. F. (760.degree. C.) to about 1700.degree. F. (930.degree. C.), and subsequently must be cooled further to a dimensionally stable condition so as to be withdrawn from the forming chamber, typically at about 1100.degree. F. (600.degree. C.). The molten metal (usually molten tin or a alloy thereof) and the atmosphere within the forming chamber are generally at a temperature lower than the glass temperature, and thus, considerable cooling of the glass takes place naturally by losing heat to the surroundings in the forming chamber. However, it is sometimes desired to employ cooling means to improve the cooling rate so as to shorten the forming chamber or to alter the temperature profile across the ribbon of glass. For these purposes, it has been common to utilize heat exchangers typically comprising metal walled conduits carrying water as the heat exchange medium. Float forming chambers also typically include heating means to help establish thermal conditions within the chamber. The use of coolers and heaters in combination to control thermal conditions within a float forming chamber are shown, for example, in U.S. Pat. No. 3,531,274 (Dickinson et al.).
It has now been found that coolers as previously employed in the art may detrimentally affect the optical quality of the glass being produced. It is believed that the sharp thermal gradient in the vicinity of a cooler may lead to undesirable viscous stresses on the top surface of the glass ribbon as it passes beneath the cooler and that the viscous stresses lead to top surface thickness variations in the glass which are evidenced as distortion of transmitted light. It would be desirable to cool the glass ribbon passing through a float forming chamber without the penalty of reduced optical quality.